Modern solar selective surface coatings use cermets (that is, metals in dielectric matrices) as absorbers of solar energy. The cermets exhibit strong absorption peaks in the main solar radiation region whilst remaining substantially transparent in the thermal (infrared) region The cermets are deposited on a bright metal infrared reflecting base to form a selective surface coating.
Selective surfaces that incorporate cermets as solar absorbers have been used on an extensive commercial scale. The most widely used are "black chrome", a graded Cr-Cr.sub.2 O.sub.3 composite produced by electroplating, and nickel-pigmented Al.sub.2 O.sub.3 produced by electrolytic colouration of anodised aluminium sheet. Surfaces such as these yield a high solar absorptance between 0.92 to 0.97 and a concomitant hemispherical thermal emittance between 0.08 and 0.26. However, the emittance of such surfaces rises steeply at high temperatures, so that the surfaces normally are useful only for low temperature applications.
Most solar collector systems have taken the form of low temperature, low concentration static systems or high concentration tracking systems. For both of these collector systems, high absorptance (ie., greater than 0.9) has a higher priority than low emitrance. In order to achieve an absorptance value greater than 0.9, graded composite absorbers have been adopted, the grading being obtained by progressively increasing the metal volume fraction and hence the refractive index from the upper level to the lower level of the absorber layer.
A graded stainless steel-carbon solar selective surface produced by DC reactive magnetron sputtering has been developed commercially and has been found to exhibit an absorptance of 0.92-0.94 and emittance of 0.04-0.05 at 100.degree. C. Also, graded Mo-Al.sub.2 O.sub.3 composite films with surface roughness have been reported as exhibiting an absorptance of 0.99 and an emittance of 0.08 at 200.degree. C.
Ungraded single cermet layers, with an isotropic metal volume fraction, deposited on a copper reflector and covered by an anti-reflection layer have been demonstrated to exhibit a normal solar absorptance of about 0.8. In particular, an absorptance of 0.87 and emittance of 0.07 has recently been reported for a 70 nm Ni-Al.sub.2 O.sub.3 cermet layer with a 0.21 Ni metal volume fraction. The cermet layer was deposited on a Mo infrared reflective layer on a Ni-plated stainless steel substrate, and was coated by an outer 60 nm thick SiO.sub.2 anti-reflection layer. Also, an emittance of 0.027 with an absorptance of 0.85 has been reported in respect of a Cr-SiO single cermet layer.
Thus, it has been established that, if a low solar absorptance can be tolerated, a very low emittance can be achieved by use of an ungraded single cermet layer. However, if film thickness or the metal volume fraction is increased, the absorption edge shifts toward longer wavelengths, increasing absorptance but, also, increasing emittance.
That is, surface coatings having a single thin cermet layer with an isotropic metal volume fraction have been shown to have a sharp absorption edge, but their absorptance is not high enough for practical solar applications. In contrast, graded composite films have been shown to exhibit absorptance greater than 0.9, but they exhibit higher thermal emittance as a result of the edge of absorption being not sharp enough, which leads dramatically to increased emitrance at operating temperatures in the region of 300.degree. C. to 500.degree. C.
The rapid rise in emitrance of graded layer coatings with temperature occurs because the thermal reradiation spectrum increasingly overlaps the spectrum of incoming solar radiation as the operating temperature of the selective absorber increases. For a high temperature selective surface, the change from low surface reflectivity in the solar spectrum to high infrared reflectivity must be as rapid as possible in order to maximise the absorptance-to-emittance ratio.